Shark Life History

Life History Trade Offs

• Energy from foraging is utilized for various physiological processes: body growth, metabolism, excretion, and reproduction.
• Energy is a limited resource; it cannot be created or destroyed, leading to trade-offs in energy allocation.
• Rapid growth can lead to increased mortality; energy invested in growth reduces lifespan.
• Organisms that grow quickly tend to mature faster but have shorter overall life spans.
• Shorter life spans often correlate with earlier reproductive strategies; organisms may reproduce sooner if their lifespans are limited.

Characteristics of Elasmobranchs

• Elasmobranchs, including sharks and rays, exhibit distinct life history traits:
  • They tend to grow into large adults and produce large pups.
  • They demonstrate low fecundity, meaning they have fewer offspring.
  • These species generally have high survival rates despite slower growth rates.
  • They reach sexual maturity later in life and tend to have longer lifespans compared to many other fish species.

Growth in Fisheries

• Individual growth refers to increases in body mass and length of fish.
• Somatic growth is crucial in fisheries management for several reasons.

Importance of Somatic Growth

• Determines biomass gain for fish stocks, impacting sustainability and resource management.
• Influences key life history characteristics like maturation, affecting breeding and population dynamics.
• Enables the estimation of other important life history parameters, such as maximum age of fish.
• Establishes a relationship between length and age, aiding in age determination for stock assessments.

Measuring Sharks

• Length serves as a reliable estimate for weight, and weight can also be inferred from length.
• Length is typically recorded along the shark's curved body and presented in centimeters (cm).
• Weight is commonly reported in grams (g).
• Total length is defined as the measurement from the tip of the snout to the end of the caudal fin.
• Precaudal length is measured from the tip of the snout to the caudal precaudal pit, which is important for certain assessments.
• Fork length is taken from the tip of the snout to the midpoint of the fork in the caudal fin, crucial for specific species identification.

Measuring Skates and Rays

• The primary measurement for skates and rays is disk width, a crucial metric for understanding their size and biology.

Length Types

• Three main length measurements are utilized: fork length (FL), precaudal length (PL), and total length (TL).

Spines and Age Determination

• Spines exhibit distinct growth bands, useful for age estimation.
• Growth bands have been confirmed to form annually in certain species, like the Spiny Dogfish (Squalus acanthias).
• The second dorsal fin is preferred for age determination due to its lesser wear compared to other fins.
• Fin spines can be challenging to analyze: they are often too large for dissection microscopy and too faint or narrow for visual counting.
• Digital imaging is recommended for spines: capture images using a standard digital camera setup, then enhance for clarity before determining age.
• Adequate lighting is crucial for successful image capture and subsequent age determination.

Release of Known Age and Marked Fish Into Wild

• Utilizes fish marked with tags or chemically through calcium binding compounds for tracking.
• This method is the most rigorous for age determination but is ineffective for long-lived species.

Bomb Radiocarbon

• Derived from the radiocarbon produced during nuclear bomb testing.
• Serves as an exceptional age validation technique for long-lived elasmobranchs (e.g., sharks and rays).
• Functions as a large-scale chemical tagging experiment, providing a unique age estimation method.
• Delta14C values establish a minimum age for samples, with 1958-1965 being the most critical period for accurate Delta14C aging.
• The low decay rate of 14C allows for both archived and currently collected samples to be used for testing.
• Costs range from 500to500 to 500to1000 per sample, with a relatively small number of samples needed for effective age validation.

Mark-Recapture Method for Fish Growth Validation

• Effective for validating growth band formation in elasmobranchs.
• Utilizes calcium-binding chemicals like oxytetracycline, alizarin, calcein, or strontium for tagging fish.
• Chemicals are injected at tagging and are absorbed into fish vertebrae and spines, creating a permanent mark.
• Marks are visible under fluorescent light and indicate the growth band formed during tagging.

Advantages and Disadvantages

• Advantage: Validates growth bands while fish grow in their natural habitat, leading to accurate growth estimations.
• Disadvantage: Low number of increments formed post-tagging may lead to significant relative error if increments are misinterpreted.

Best Practices for Robust Validation

• Tagging young fish and recapturing them at an older age yields the most reliable data on growth validation.

Application and Effectiveness

• Considered a powerful validation method alongside bomb radiocarbon dating for adult wild elasmobranchs.

Somatic Growth in Sharks

• Age determination in some shark species can be complicated by irregular vertebral formation, decay, or lack of annual growth bands, leading to potential mismanagement.
• Alternative methods such as mark-recapture tagging provide insights, yet they are limited by biases like growth variability among individuals, short library time, small sample sizes, or presence of outliers.
• Von Bertalanffy growth curves are utilized to estimate age based on length data, reflecting the average growth rate of shark populations.
• Growth rate is not uniform; it decreases as the length of the shark increases.
• Elasmobranchs, which include sharks, continue to grow throughout their entire lives, exhibiting indeterminate growth.
• Accurate age determination of elasmobranchs is particularly challenging, and errors in age assessment can lead to biased growth curves.

Reproduction in Sharks

• Life history theory suggests optimal paternal investment enhances offspring survival and future reproductive success.
• Maternal survival and reproductive potential balance against the level of paternal care provided.
• Sharks exhibit minimal parental care, relying on an enlarged liver and yolk sac for nourishment, with optimal pupping conditions.
• Shark offspring are independent at birth, exhibiting relatively large size and high survival rates.
• Gestation periods for sharks range from 6 to 36 months, with an average of 12 months.
• The reproductive cycle, or inter-birth interval, typically spans 1-3 years for female sharks.
• Litter sizes vary dramatically from 1 to 300 pups, with an average of 13 pups per litter.
• Data on size and age at maturity are often presented as the lengths and ages when 50% of individuals reach maturity, analyzed through logistic regression.
• A positive correlation exists between maternal size and both the size and number of offspring produced, indicating increased fecundity with larger females.
• Experience (related to food availability) and age (size) of the female are significant factors influencing reproductive outcomes.
• Certain shark species may experience a trade-off between increasing litter size and the mass of individual offspring.
• Understanding these reproductive traits is crucial for conservation and management strategies in shark populations.
• Fecundity shows a direct increase with maternal length, impacting population dynamics and sustainability.

Longevity in Species

• Longevity refers to the maximum age recorded for different species, providing insight into their lifespan potential.
• It indicates the maximum time available for an organism's growth and reproduction, influencing population dynamics.
• Longevity serves as an effective proxy for assessing natural mortality rates within species populations.
• Mathematical approximation of longevity indicates that it correlates to approximately 4.6 years per k (where k represents growth rate).

Causes of Natural Mortality

• Natural mortality encompasses various factors that lead to fish deaths, including:
  • Predation: Other species consuming them as food.
  • Disease: Illnesses that can lead to mortality among fish populations.
  • Parasites: Organisms that live on or in the fish, potentially causing harm or death.
  • Accidents: Unintentional events leading to death, such as getting caught in debris.
  • Natural Death: General aging processes leading to mortality.
  • Old Age: The eventual decline in health and vitality associated with aging.

Life History Trade Offs

• Foraging provides energy that is allocated to various biological functions including growth, metabolism, excretion, and reproduction.
• Energy allocation is crucial as it cannot be created or destroyed, leading to inherent trade-offs in life history strategies.
• Resources obtained through foraging must be carefully distributed among different life processes to maximize survival and reproductive success.
• The balance between somatic growth (body size and strength) and reproductive output is a key consideration in life history strategies.
• Trade-offs may result in prioritizing growth over reproduction or vice versa, depending on environmental conditions and species-specific life strategies.